Electronics > Projects, Designs, and Technical Stuff

Trap for Young Players: HF Reject

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Prehistoricman:
I was at my university's electronics lab today working on some reverse-engineering/glitching attack when I realised that my scope was triggering in completely the wrong place.
i had an microcontroller set up to deliver a clock burst (green) and before sending it, it would set an IO line high (my trigger). However, what I realised was that I was triggering way after the burst.



Turns out this HF reject option delays the trigger significantly.
Here is with HF reject turned off:



Why do you think Keysight didn't adjust the trigger in software to account for the filter delay?

David Hess:

--- Quote from: Prehistoricman on December 17, 2019, 01:16:20 am ---Why do you think Keysight didn't adjust the trigger in software to account for the filter delay?
--- End quote ---

I suppose they could since a given filter has a fixed group delay but analog oscilloscopes do the same thing when high frequency reject is used.  When high frequency reject is used appropriately on low frequency signals, the delay is insignificant compared to the time/div.

T3sl4co1l:
What delay?  If it's triggering on Ch1, it looks to be triggering on the step change, rounded over slightly as you would expect given the bandwidth specified for "HF reject".

It would be more obvious if they had a "trig view" function, though you may have a software filter function that can approximate such a display.  If so, try it with different signals, combinations of transients/glitches, pulses, bursts and steps, and get a feel for what it's doing. :)

Tim

David Hess:

--- Quote from: T3sl4co1l on December 17, 2019, 04:08:13 am ---What delay?  If it's triggering on Ch1, it looks to be triggering on the step change, rounded over slightly as you would expect given the bandwidth specified for "HF reject".
--- End quote ---

But the trigger point is misaligned by about 600 nanoseconds.

On an analog oscilloscope if you make that mistake, it often becomes very apparent because the displayed jitter is much higher.

jklasdf:
It looks like you're triggering on the Ext. input, I'd be interested in seeing what it looks like.

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